The idea is good but involved a lot of money and if the exchange you're planning to create is no KYC/P2P.  It will be easy for you to have a good reputation if create an expensive site, have good customer support, a fast deposit/withdrawal feature, a marketable Ann thread, and have a signature campaign advertisement on this forum.
Once you have a good reputation on this and also do some marketing outside the forum it is easy for the exchange to get the needed trust, traffic, and reputation.